1. Field of the Invention
This invention relates to a dry etching method employed in the preparation of semiconductor devices. More particularly, it relates to a method for low temperature anisotropic etching of a layer of a silicon-based material, such as a single crystal silicon, polysilicon or refractory metal silicide, at a mid to low temperature range without employing chlorofluorocarbon (CFC) based gases.
2. Description of the Related Art
In keeping up with the tendency towards a higher integration and a higher performance of semiconductor devices, as exemplified by VLSIs and ULSIs of recent origin, a strong demand is raised for a technique whereby requirements for high anisotropy, high etchrate and high selectivity may be unanimously met in the field of etching of a layer of a silicon based material, such as a single crystal silicon, a polysilicon, a refractory metal silicide or polycide.
A representative etching process for single crystal silicon is trench etching, that is processing for forming a trench with view to isolation of fine devices or procurement of a cell capacity area. Although it is required of this process to effect anisotropic etching with a pattern of a high aspect ratio, the cross-sectional profile of a trench tends to be changed intricately due to fluctuations in the mask pattern or changes in the etching conditions, so that unusual profiles such as undercuts or bowing are frequently encountered. These give rise to difficulties in trench filling and capacity control during the subsequent process.
On the other hand, a representative etching process for polysilicon or polycide is gate processing. The pattern width of the gate electrode directly affects the channel length of a transistor when its source/drain region is formed in a self-aligned manner or the dimensional accuracy of the sidewall in an LDD structure. Hence, extremely high processing accuracy is also required of this etching process.
Conventionally, CFC gases, exemplified by CFC113 (C.sub.2 Cl.sub.3 F.sub.3), were used extensively as an etching gas for etching the silicon-based material. Since the CFC gases contain F and Cl in its molecule, etching may proceed in accordance with the radical reaction by radicals such as F* or C1* and the ion assisted reaction by ions such as CF.sub.x.sup.+, CCl.sub.x.sup.+ or Cl.sub.x.sup.+, depending on conditions. High anisotropy may be achieved while there proceeds sidewall protection by a carbonaceous polymer deposited from a gaseous phase.
However, the CFC gas has been pointed out as causing destruction of an ozone layer of the earth and a ban will be placed on the production and application thereof in near future. Thus it is incumbent in the field of dry etching to find a substitute material for CFC gases and a method for using the material effectively. On the other hand, with further refinement of the design rule of the semiconductor devices in future, it may be feared that the carbonaceous polymer deposited from the gaseous phase will prove to be a source of pollution by particles. For this reason, post-CFC gas measures are strongly desired.
Among the techniques thought to be promising as post-CFC gas measures is a low-temperature etching, according to which the wafer temperature is maintained at a temperature not higher than 0.degree. C. to maintain the etchrate along the depth at a practical level under the ion assisted effect while the radical reaction on the pattern sidewall is frozen or suppressed to prevent the occurrence of unusual shapes such as undercuts. In an Extended Abstracts of the 35th Spring Meeting (1988) of the Japan Society of Applied Physics and Related Societies, page 495, title number 28a-G-2, a report has been made of an example of cooling a wafer to -130.degree. C. and effecting silicon trench etching and etching of an n.sup.+ type polysilicon layer.
Meanwhile, if high anisotropy in low temperature etching is to be achieved solely by freezing or suppression of a radical reaction, cooling to a lower temperature lower than -100.degree. C. is required so that it becomes mandatory to provide a cooling system for liquid nitrogen circulation in the etching system. This means that the hardware is increased in size and expensive, while it becomes difficult to maintain high reliability of O-rings or vacuum seals employed in a vacuum system under such low temperature. There is also presented a problem that the cooling time or the time necessary in resetting the wafer to room temperature is prolonged to lower the throughput. For this reason, for introducing low temperature etching into a mass-production process, it has been desired to develop a process which can be carried out at a low to mid temperature of -100.degree. to 0.degree. C. and preferably -70.degree. to 0.degree. C. which can be achieved by using a simpler cooling unit such as a chiller or a CFC based cooling medium, such as "Fluorinert" manufactured and sold by SUMITOMO 3A Co. Ltd. For realizing high anisotropic processing in such temperature range, it is thought to be a practical approach to combine the suppression of the radical reaction at a lower temperature and sidewall protection by deposits.
As such approach, the present inventor has previously proposed a technique which resides in low temperature etching of a layer of a silicon oxide based material at a temperature in the vicinity of -100.degree. C. using sulfur fluorides such as S.sub.2 F.sub.2 as an etching gas. This technique makes use of one of several sulfur fluoride gases which has an S/F ratio higher than that of well known SF.sub.6, wherein the S/F ratio means a ratio of the S atoms to F atoms in a molecule. With this technique, the amount of the F* radicals in the etching system may be reduced while sulfur may be deposited on the pattern sidewall. That is, silicon selectivity may be improved by decreasing the amount of F* and the effects of sidewall protection may be achieved by S deposition. On the other hand, the temperature of achieving anisotropy may be drawn closer to room temperature than when SF.sub.6 is used alone. Moreover, deposited S may be easily sublimed off by heating the wafer on completion of etching, so that there is no risk of pollution by particles.
The present Assignee has proposed a technique of low temperature etching of a polycide film at a temperature in the vicinity of -50.degree. C. by a mixed gas containing sulfur fluoride having a high S/F ratio such as S.sub.2 F.sub.2 and HBr. As for etching of the polycide film, unusual shape such as undercuts is frequently produced in the lower doped polysilicon layer which is susceptible to attack by radicals. However, with this technique, since both SiBr.sub.x and S contribute to sidewall protection, good shape anisotropy may be maintained by S deposition even if Br* (bromine radicals) were in excess during overetching.
The present inventor has also proposed in Digest of Papers, 1991 4th MicroProcess Conference, page 32, A-3-1, a technique of low temperature etching using a gas containing sulfur halides, such as S.sub.2 Cl.sub.2 or S.sub.2 Br.sub.2. This is aimed at employing a gas which is incapable of producing reactive F* for diminishing the effects of radicals.
It will be seen from the above that the techniques of effecting sidewall protection by S is highly promising because a clean process may be provided as long as etching of the layer of the silicon oxide based material is concerned.
However, many problems remain to be solved in applying the present technique to the etching of the layer of the silicon-based material, because the layer of the silicon-based material is highly susceptible to attack by radicals. Even with the use of the compound such as S.sub.2 F.sub.2 having the highest S/F ratio among sulfur fluorides, F* are still in excess and unusual shapes such as undercuts are frequently produced under the mask. The effects of the reactive F* may be diminished by adding HBr to S.sub.2 F.sub.2 as mentioned previously. However, addition of Br into the etching reaction system raises another problem. That is, if reaction products exhibiting low vapor pressure, such as SiBr.sub.x or WBr.sub.x as a result of etching of tungsten silicide, dimensional loss or pollution by particles may be feared to take place as a result of excess deposition of these reaction products. Similar problems are caused by using sulfur halides not producing F*.
As a problem inherent in the above mentioned techniques, the above sulfur halides, having the low S/X ratio, where X stands for a halogen atom, are not as yet mass-produced as dry etching gases. It is therefore desirable to use other less expensive commercially available gases.